An electrical power distribution system comprises distance protection systems arranged to protect, monitor and control the functioning of devices forming part of the power distribution system. The protection systems prevent, among other things, short-circuits, over-currents and over-voltages in power lines of the power distribution system.
Protective relays are used throughout the electrical power distribution system for providing such protection and control. The protective relays detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers, and they are operated through current transformers (CT) arranged to sense a fault current and act accordingly. Briefly, a fault direction can be determined based on observations of the voltage and current conditions in the power distribution system. The current and voltage waveforms are analyzed in order to determine the impedance. If the impedance is above or below a certain value, it can be established whether a fault is present or not. Further, the direction of the fault may also be determined, for example by determining the angle between the phase of the voltage and the phase of the current.
In a breaker-and-a-half system the current signals from two sets of current transformers are summed to obtain the current that is going out on a power line. If there is an external fault present with high currents through the two current transformers, then one of them might saturate. Depending on which one of the current transformers gets saturated, a wrong directional decision can be made by the protective relay.
Further, when a current transformer saturates its inductance drops and there is then a risk to harm other load devices, besides the above-mentioned risk of making incorrect directional decisions. Care has to be taken to avoid such core saturation and the current transformer specifications should be adequately chosen in dependence on their intended use.
Presently such saturation problems are ignored. There are thus no fault direction detection algorithms and devices for combating saturation of current transformers.
In view of the above, it would be desirable to provide an improved protective relay for handling saturation of the current transformers. Further, it would also be desirable to provide a corresponding method.